A golf ball generally has an infinite number of dimples. In a case of a golf putter having a putter head face (hereinafter may be referred to as “putter face”) made of metal or a composite that is sufficiently high in rigidity as compared to a rigidity of the golf ball and its surface, a ball contact surface of the putter face (hereinafter may be referred to as “face surface”) and the golf ball has point contact, particularly in a short putt or a descending line.
Note that the rigidity is generally represented by modulus of elasticity (Young's modulus), shear modulus of elasticity, and the like. In a point of comparison in physical property values, the higher the rigidity the more difficult for a shape to change, and the lower the rigidity the softer the material.
As described above, when the face surface and the golf ball has point contact, particularly when an edge part of the dimples on the golf ball has point contact with the face surface, the golf ball rolls in a trajectory off a target direction perpendicular to the face surface, due to a shift of a rebounding direction caused by the edge part of the dimple. For example, in theory, a problem occurs such that a disposition of 16 mm to 22 mm occurs 1.2 m away from the face surface.
In order to solve such a problem, there have been reference techniques in which a depressed section is formed on one part of the face surface of the putter head, so as to insert a face made of material having a low rigidity, such as resin material. This allows the ball contact surface of the face to change its shape along a spherical surface of the golf ball.
One example of such reference technique is a golf putter 100 disclosed in Japanese Unexamined Patent Publication No. 224716/2001 (Tokukai 2001-224716; published on Aug. 21, 2001).
In FIG. 15, (a) is a front view of a golf putter 100 from a face surface side, and (b) is a side view of the golf putter 100 having the face surface side as front.
The golf putter 100 is arranged so that a head 102 is attached to a tip section of a shaft 101, as illustrated in (a) and (b) of FIG. 15.
In addition, a depressed section is formed on a part of a face surface 103 of the head 102, and a face plate 104 is assembled on this depressed section.
Further, the face plate 104 is made of resin material which has a surface hardness of a JIS (Japanese Industrial Standard)-D in a range of 40 to 60, a resilience in a range of 30% to 60%, and a coefficient of kinetic friction on a surface in a range of 0.4 to 2.0. Such the face plate 104 thus allows the golf putter 100 to attain a soft touch, a good sense of distance, and good control performance at the time when striking a ball.
The surface hardness of the JIS-D less than 40 causes the face plate 104 to be excessively soft, and the surface hardness of the JIS-D more than 60 makes it impossible to attain the soft touch at the time when striking the ball.
The resilience not within the range of 30% to 60% causes difficulty in attaining an aimed distance.
Furthermore, the coefficient of kinetic friction less than 0.4 causes a direction of the ball thus struck to be unstable, due to a quick separation of the ball from the face plate 104.
Other than the above, a U.S. patent (U.S. Pat. No. 6,699,140B1; patented on Mar. 2, 2004) discloses a putter face composed of three layers of soft polymer.
In addition, a U.S. patent (U.S. Pat. No. 4,679,792; patented on Jul. 14, 1987) discloses a putter face having a plurality of independent cells of an elastic body made of epoxy resin, which plurality of independent cells construct a honeycomb structure.
The golf putter 100 is inserted with the face plate 104 whose ball contact surface is made of soft material (resin), therefore the face elastically deforms along a spherical surface of the ball. This prevents occurrence of effects caused by the edge part of the dimples. However, loss of energy at a time of impact is great as compared to a hard metal face.
Therefore, no matter how the material of the face is prepared, the golf putter 100 drops in an initial velocity as compared to the hard metal face. As a result, a problem occurs such that a sufficient touch at a time when striking the ball (hereinafter referred to as “striking touch”) and a sufficient sense of distance (rolling of the ball in accordance with the touch) are not attained.
It is believed that a sense of sound of an impact sound that is produced when the ball and the putter face collide with each other subtly influences this striking touch. Therefore, it is assumed that for intermediate and advanced players of golf, this subtle difference in the sense of sound influences their playing performance.
That is to say, it is expected that although a beginner of golf would preferably use the golf putter 100 attainable of a good sense of distance and good control performance while having a soft touch, the intermediate and advanced players of golf may feel unsatisfied with the golf putter 100 due to this subtle difference in the sense of sound.
Namely, it is considered that for the intermediate and advanced players of golf, the impact sound which is produced in collision of the ball and a putter face having a higher rigidity than the ball (for example, a putter face made of metal) is an important factor which influences their playing performance.
The putter face described in the U.S. patent (U.S. Pat. No. 6,699,140B1; patented on Mar. 2, 2004) consists of three layers of soft polymer. This putter face simply piles three layers of elastic bodies. Although this putter face elastically deforms so as to prevent the effects caused by the edge part of the dimples, it is believed that the loss of energy at the time of the impact is still high, as compared to the hard metal face.
Further, the putter face described in the U.S. patent (U.S. Pat. No. 4,679,792; patented on Jul. 14, 1987) has a plurality of independent cells which construct a honeycomb structure. Each of the cells is an elastic body made of epoxy resin, therefore has a strong frictional force between the cells. As a result, the plurality of cells is elastically deformed in connection with each other. Consequently, although the putter face is constructed of the plurality of independent cells, functions of the putter face are hardly different to the putter face having an elastic body inserted therein as disclosed in the Japanese Unexamined patent publication No. 224716/2001 (Tokukai 2001-224716; published on Aug. 21, 2001) and the U.S. patent (U.S. Pat. No. 6,699,140B1; patented on Mar. 2, 2004). Therefore, although the effect caused by the edge part of the dimple is prevented, the loss of energy at the time of the impact is high, as compared to the hard metal face.